Precision mounting of components can present certain challenges. Many high precision systems require adjustability of the position or angle of a component, as well as stability once the component is in place. Such systems may include but are not limited to optical systems. For example, it may be desirable to adjust “tip,” “tilt” and “piston” of a component, with “tip” and “tilt” meaning angular adjustment of the component in orthogonal axes, and “piston” meaning translational adjustment of the component in and out of a plane formed by the axes. FIG. 1 is a schematic drawing of a mechanical assembly 1 that includes a component 2 mounted with a frame 3. An axis 10 may be designated the tilt axis, axis 20 may be designated the tip axis, and arrow 30 indicates the direction of piston movement. A set of XYZ axes is also shown in FIG. 1, with rotation about each of these axes designated by two of the axis symbols (e.g., rotation about the X axis is designated as the XX direction). Thus, in FIG. 1, tilt is equivalent to a YY rotation of component 20 centered about the component; tip is equivalent to an XX rotation of component 20 centered about the component, and piston is equivalent to translation of component 20 in the Z direction relative to frame 3.
Certain components such as optical components are often fabricated of materials such as glass, while structural components are often fabricated of metal. Since glass and metal have different coefficients of thermal expansion (“CTE”), the structural components may place stress on the optical components when temperature changes. Stress can, in turn, lead to deformation of optical surfaces. Certain optical systems include components having surfaces fabricated within tolerances of fractions of a wavelength; such systems may degrade in performance when temperature changes impart even small stresses on the components.
One strategy for mounting components with high stability precision and adjustability while mitigating degradation over a temperature range is to mount the component utilizing flexures. When flexures are used, positional adjustments may be made to the mounting of the flexure rather than to the component itself. Flexures may be mechanically weaker than the components mounted therewith, so that temperature changes cause deformation of the flexure instead of the component. However, certain flexure arrangements may be problematic in that they respond inappropriately to mechanical forces and/or are complex to build and install. For example, certain flexure arrangements may exhibit mechanical resonance in response to vibrations, and/or may permit components mounted therewith to move laterally or to twist in response to impulse forces. Other flexure arrangements utilize complex positioning schemes or actuators that may introduce issues such as high cost, tight mechanical tolerances of individual parts to produce acceptable tolerance stack-ups, and degraded performance when moving parts incur wear.
Another way to mount optical components with high precision and adjustability is through the use of complex assemblies such as gimbal mounts, which are also formed of multiple parts and which introduce many of the same issues.
An ideal mount, from a mechanical engineering perspective, is sometimes called a kinematic mount, and is characterized by its ability to constrain motion of a component in six degrees of freedom (e.g., the X, Y, Z, XX, YY and ZZ directions, as shown in FIG. 1) without overconstraining motion of the component. Overconstraining means that displacement of the mounted object in an axis of any of the six degrees of freedom generates not only an opposing force along the same axis, but also generates a force along or about one or more other axes. An advantage of a kinematic mount is that adjustments to a position of a component mounted therewith can be made (1) in one direction at a time, without affecting position or rotation of the mounted component in other directions, and (2) without introducing unintended forces on the mounted component (e.g., forces that could cause distortion of precision components).
A stiffness matrix may be used to characterize performance of an assembly. The stiffness matrix quantifies the reaction of the assembly to forces acting on it. Each row in the stiffness matrix represents reaction to either a displacement along one of three orthogonal axes or a rotation applied about one of three orthogonal axes. An entry in each column of the stiffness matrix shows the opposing, responsive force generated by the assembly in response thereto. When a component mounted with a kinematic mount is characterized by its stiffness matrix, all of the non-diagonal terms of the stiffness matrix are zero; that is, the mounted component generates an opposing force exactly equal in type to an applied displacement or rotation, without generating any force in or about other axes. For example, in a kinematic mount, a displacement along an X-axis would generate an opposing force in the X-axis without generating any force in Y- or Z-axes, or rotational force in any of the XX, YY or ZZ directions. Another way of characterizing a kinematic mount is to say that any physical constraints applied to the mounted object are non-redundant.